R devel - May 1997 - R-alpha: R-0.49 / S-plus: "default argument evaluation" bugs and woes

There is a problem with  'default argument evaluation'
when I use an existing function name as argument name :

sintest <- function(x, y = 2, sin= sin(pi/4))
{
  ## Purpose: Test of   "default argument evaluation"
  ## -------- Fails for  R-0.49.  Martin Maechler, Date:  9 May 97.
  c(x=x, y=y, sin=sin)
}

## R-0.49:
R> sintest(1)
##> Error in sintest(1) : recursive default argument reference

## S-plus 3.4  (being 100% ok):
S> sintest(1)
  x y       sin 
  1 2 0.7071068
 Warning messages:
   looking for function "sin", ignored local non-function in:
sintest(1)

-------------------------------------------------------
The following shows bugs, both in R and S:

sintest2 <- function(x ,y = 2)
{
  ## Purpose: Test of   "default argument evaluation"
  ## -------- Fails for  S-plus 3.4.  Martin Maechler, Date:  9 May 97.
  c(x=x, y=y, sin=sin)
}

R> sintest2(1)
[[1]]
[1] 1

[[2]]
[1] 2

[[3]]
<primitive: sin>

--------------- is almost okay, 
		the buglet being that the names have been dropped from the list.

But watch this:
S> sintest2(1)
function(x = 1, y = 2, sin.x)
sin2 = .Internal(sin(x), "do_math", T, 109)

--- returning a function 

	((now we see, why S's way of treating functions as
	  lists sometimes badly sucks)).
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Martin Maechler wrote:
> 
> 
> There is a problem with  'default argument evaluation'
> when I use an existing function name as argument name :
> 
> sintest <- function(x, y = 2, sin= sin(pi/4))
> {
>   ## Purpose: Test of   "default argument evaluation"
>   ## -------- Fails for  R-0.49.  Martin Maechler, Date:  9 May 97.
>   c(x=x, y=y, sin=sin)
> }
> 
> ## R-0.49:
> R> sintest(1)
> ##> Error in sintest(1) : recursive default argument reference
> 
> ## S-plus 3.4  (being 100% ok):
> S> sintest(1)
>   x y       sin 
>   1 2 0.7071068
>  Warning messages:
>    looking for function "sin", ignored local non-function in:
sintest(1)
> 
> -------------------------------------------------------
> The following shows bugs, both in R and S:
> 
> sintest2 <- function(x ,y = 2)
> {
>   ## Purpose: Test of   "default argument evaluation"
>   ## -------- Fails for  S-plus 3.4.  Martin Maechler, Date:  9 May 97.
>   c(x=x, y=y, sin=sin)
> }
> 
> R> sintest2(1)
> [[1]]
> [1] 1
> 
> [[2]]
> [1] 2
> 
> [[3]]
> <primitive: sin>
> 
For better or worse, S and R allow default expressions to contain
references variables that are (or rather may be) created in the
function body, so (in R and Splus)
> x<-1
> f<-function(a,b=x) { if (a) x<-2; b}
> f()
Error: Argument "a" is missing, with no
default
> f(T)
[1] 2
> f(F)
[1] 1

More traditional lexical scoping would make the reference to x in the
default always be global, but lots of code would break. I think we're
stuck with this behavior as a corollary to the way S wants default
arguments to work.

Actually S is a bit inconsistent in its error message -- if you have a
non-function argument it gives the same message as R,
> g<-function(x=x) x
>g()
Error in g(): Recursive occurrence of default argument "x"
Dumped

Also in R's lexical scoping you probably do want the argument name to
shadow any outer definitions if you want to be able to define default
arguments that are recursive functions, e.g.
> g<-function(n, nfac=function(x) { if (x <= 1) 1 else nfac(x-1)*x })
nfac(n);
> g(6)
[1] 720

-- 
Luke Tierney
University of Minnesota                      Phone:           612-625-7843
School of Statistics                         Fax:             612-624-8868
206 Church Street                            email:      luke@stat.umn.edu
Minneapolis, MN 55455 USA                    WWW:  http://www.stat.umn.edu
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lm.influence() doesn't work correctly on weighted models (and thus on glm
objects).


Thomas Lumley
------------------------------------------------------+------
Biostatistics		: "Never attribute to malice what  :
Uni of Washington	:  can be adequately explained by  :
Box 357232		:  incompetence" - Hanlon's Razor  :
Seattle WA 98195-7232	:				   :
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